Burner

ABSTRACT

A burner for combusting gaseous mixture of gaseous fuel with a combustion supporting gas, such as oxygen or air, comprising a burner tube ( 11 ) open at one end ( 11 ′) and closed at its other end ( 11″ ) with a flame holder ( 30 ) at which fuel is burnt adjacent the open end ( 11 ′), the flame holder ( 30 ) being traversed by passageways ( 52, 54, 56, 58 ) for the gaseous mixture, the burner ( 10 ) having inlets ( 14, 16 ) adjacent the closed end ( 11 ″) connected to combustion supporting gas and gaseous fuel supply lines, one of said lines having a control valve operable for controlling the size of the flame, the said one line having a pressure or flow transducer and the other line having a variable booster or restricter responsive to the transducer, for balancing air and fuel supplied to the burner ( 10 ) to ensure the gaseous mixture remains stoichiometric irrespective of the size of the flame and such that the lowest gaseous fuel mixture flow rate is at least as low as {fraction (1/60)}th the highest flow rate of the gaseous fuel mixture each passageway ( 52, 54, 56, 58 ) having a flared exit ( 60 ) at the end nearer the open end ( 11 ′) of the burner ( 11 ) each passageway being dimensioned such that at the highest obtainable flow rate of gaseous fuel mixture the flames do not lift off from the flamer holder, at the lowest flow rate the velocity of the gaseous fuel mixture at some point within the passageway ( 52, 54, 56, 58 ) is sufficient to prevent flame back though the flame holder.

The present invention relates to a gas burner suitable for use inincinerators, boilers, space heating appliances and ovens, furnaces orhigh temperature reactors used in industry, for example. A burnerincorporating the flame holder is also highly suitable for use in aflare stack.

The gas to be used as fuel can be any of the combustible gases commonlyused in gas burners. For example, the gas can be butane, propane,natural gas and hydrocarbon product gases produced by gasification oforganic materials, such as commercial or general domestic waste.

The burner disclosed hereinafter has been devised to secure completemixing of the fuel and air or oxygen, and to admit them to a mixingchamber in the burner only in the correct stoichiometric ratio requiredby the fuel for its complete combustion whilst providing a stable flameover a turn-down ratio of up to 60:1, at least.

A preferred burner for combusting gaseous fuel, comprises a burner tubeopen at one end and closed at its other end with a flame holder at whichfuel is burnt adjacent the open end, the flame holder being traversed bypassages for fuel and air to be consumed, the burner having inletsadjacent the closed end respectively for air, or oxygen, and fuel, theinlets being furnished with metering nozzles for separately deliveringair and fuel substantially radially into the tube which forms a mixingzone between the inlets and the flame holder, the metering nozzleshaving orifices with flow cross-sectional areas correlating to thestoichiometric ratio of air-to-fuel for which the fuel is substantiallycompletely burnt.

A burner of the present invention beneficially tolerates widely-varyingair/fuel flow rates, i.e. it has a high turn-down ratio. Conventionalburners have turn-down ratios of the order of 4 or 5 to 1. Thus, thesupply rates of air and fuel can be reduced to one quarter or one fifthof the maximum capacity of such burners. Further reduction results inflame instability; ultimately the flame fails and is extinguished.

The present invention seeks to provide a burner with a much larger turndown ratio. Accordingly, it provides a burner for combusting gaseousmixture of gaseous fuel with a combustion supporting gas, such as oxygenor air, comprising a burner tube open at one end and closed at its otherend with a flame holder at which fuel is burnt adjacent the open end,the flame holder being traversed by passageways for the gaseous mixture,the burner having inlets adjacent the closed end connected to combustionsupporting gas and gaseous fuel supply lines, one of said lines having acontrol valve operable for controlling the size of the flame, the saidone line having a pressure or flow transducer and the other line havinga variable booster or restricter responsive to the transducer, forbalancing air and fuel supplied to the burner to ensure the gaseousmixture remains stoichiometric irrespective of the size of the flame andsuch that the lowest gaseous fuel mixture flow rate is at least as lowas {fraction (1/60)}^(th) the highest flow rate of the gaseous fuelmixture each passageway having a flared exit at the end nearer the openend of the burner each passageway being dimensioned such that at thehighest obtainable flow rate of gaseous fuel mixture the flames do notlift off from the flamer holder, at the lowest flow rate the velocity ofthe gaseous fuel mixture at some point within the passageway issufficient to prevent flame back through the flame holder.

The burner of the present invention represents a marked departure fromprior art burners in that the burner can provide a stable flame at theflame holder at low flow rates yet can provide a 60 fold increase ingaseous mixture flow rate by providing sources of gaseous fuel andcombustion supporting gas which can provide sufficiently high pressuresto provide, at the high flow rate, a sufficient pressure drop over theflame holder passageways to obtain the required flow rate.

The burner of the present invention holder of the can provide aturn-down ratio of the order of 60:1, and thus a stable flame isretained even when the supply of air and fuel is reduced to one sixtiethof the maximum capacity.

Such a high turn-down ratio is highly advantageous, since heat outputcan be controlled over a wide range. Moreover, such a burner is idealfor use in situations where the gas supply is variable, such as mayoccur in the case of flare stacks.

The inlets may be furnished with metering nozzles for separatelydelivering air and fuel non-axially, e.g. substantially radially intothe tube which forms a mixing zone between the inlets and the flameholder the metering nozzles having orifices with flow cross-sectionalareas correlating to the stoichiometric ratio of air-to-fuel for whichthe fuel is substantially completely burnt. Preferably the inlets aredisposed in the tube for delivering air and fuel in directions whichimpinge, to create turbulence and mixing inside the tube, for example bylocating the inlets diametrically opposite one another in the tube.

Conveniently, the flame holder provides a mounting for an igniter andassociated ground electrode, and, optionally, further provides amounting for an ionization probe.

Preferably the burner includes a monitor and control system coupled tothe probe, for interrupting the fuel supply should the unburnt carbonexceed a predetermined level.

In such an embodiment, there may be a valve in the air supply line and abooster or restricter in the fuel line, or there may be a valve in thefuel line and a variable speed fan provided in the air line.

The flame holder may comprise two or more radially nested tubes eachpair of adjacent tubes defining therebetween one of said passageways ofthe flame holder for the gaseous fuel, but other ways of defining thepassageways may be employed, for example, a plurality of holes in adisc.

The tubes (30 a, 30 b, 30 c) may be held in position relative to eachother by one or more transverse pins (33) and include a central borewith a flared exit.

Each flared exit may have its terminal portion defined by inner andouter cylindrical walls which are parallel to the longitudinal axis ofthe flame holder.

A burner of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is an end view of the burner incorporating an embodiment of flameholder according to the present invention;

FIG. 2 is a longitudinal cross-section through the burner of FIG. 1, online II—II of FIG. 1;

FIG. 3 is a longitudinal cross-section of the flame holder end of theburner of FIG. 1 on line III—III of FIG. 1; and

FIG. 4 is a schematic diagram of the burner of FIG. 1 in circuit withflame size control apparatus.

The burner 10 illustrated in the drawings comprises a tubular case 11 ofheat resistant material such as stainless steel, and is provided with amounting flange 13 for securing it in a combustor apparatus, not shown.The combustor apparatus could be a boiler, a gas-fired space heatingappliance, a furnace, or a flare stack, for example.

A forward end 11′ of the burner is open, for the flame to issuetherefrom, and the opposite, rearward end 11″ is closed by and sealed toan acrylic viewing window 12.

Adjacent the rearward end, there are inlets 14, 16 for air (or oxygen)and for fuel, i.e. combustible gas. The inlets 14, 16 are internallyscrew-threaded to receive unions for coupling them to appropriateair/fuel supply lines.

The fuel inlet 16 is smaller than the air inlet 14. Both inlets 14, 16are internally screw-threaded and inside each is a metering nozzle 18,20. Metering nozzle 18 has a bore 22 which is of substantially greaterdiameter than bore 24 of metering nozzle 20.

The flow cross-sectional areas of the bores 22, 24 are in a ratiocorresponding to the stoichiometric ratio of fuel-to-air, at which thecombustible fuel is completely oxidized, i.e. burned. For completecombustion, different fuels require different amounts of air (oroxygen), and hence the stoichiometric ratios will vary from one fuel toanother.

It is contemplated, therefore, that the nozzles 18 and 20 will bematched to the stoichiometry requirements of the particular fuel to becombusted. Thus, one or both nozzles 18, 20 will be changed to suit thefuel, whenever the fuel to be combusted is changed, to maximisecombustion efficiency, the gases being supplied to the nozzles 18 and 20at the same pressure so the flow of fuel and air is proportional to thebores of 22, 24 of the nozzles 18, 20 and which equal pressure conditionwill be assumed for the remaining description.

The required ratio of the flow cross-sectional areas of bores 22, 24 canbe determined empirically. Alternatively, it can be establishedtheoretically if the composition of the fuel is known.

By way of example, the ratio of the areas of bores 22, 24 is of theorder of 10:1 for fuels comprising hydrocarbon gas mixtures, at air andgas pressures of the order of 30″ water gauge (76 mbar). By way ofcomparison, existing high pressure burners may operate at 2-3″ watergauge (5.1-7.6 mbar). Standard commercial burners are usually run at0.5″ water gauge (1.3 mbar) air pressure and 2″ water gauge (5.1 mbar)gas pressure.

Inside the burner case 11 there is a fixed flame holder 30 according tothe present invention fabricated from nested coaxial steel rings. Theflame holder 30 defines basically annular jets from which streams ofmixed fuel and air issue. The jets are ignited to establish the requiredflame. To ignite the jets, a spark igniter is provided. The ignitercomprises a spark electrode 32 and a ground electrode 34. The electrode32 is electrically insulated from the flame holder 30. The electrodes 32and 34 extend rearwardly to and through the window 12 to respectiveterminals 36, 38 for connection to an electrical supply.

The flame holder 30 is made up of three coaxial tubes 30 a, 30 b, 30 cheld in a fixed spatial relationship by axially spaced, transverse brasspins 33 (see FIG. 3) which have been push fit in aligned diametric holesthrough the tubes 30 a, 30 b, 30 c. The flame holder 30, as a unit, issupported and located within the burner case 11 by pins 31.

The tubes 30 a, 30 b, 30 c are dimensioned and configured to providerelatively narrow annular passageways 52, 54, 56 and 58 between the tube30 a and the burner case 11 between tubes 30 a and 30 b, between tubes30 b and 30 c and between tube 30 c and electrode 30. All thesepassageways have flared exits 60 at the end of the flame holder 30nearer the open end 11′ of the burner tube 11.

Three tubes are present in the illustrated embodiment but the numberselected, from one upwards, is determined by the maximum power outputrequired from the burner 10.

Each of the tubes 30 b and 30 c has a pair of longitudinalhalf-cylindrical grooves which co-operate to provide two generallycylindrical passages for insertion and retention of the electrode 32 andprobe 40, as shown in FIG. 1, the remainder of the annular passagebetween tubes 30 b and 30 c being as provided between tubes 30 a and 30b as can be seen in FIG. 3.

The passageways and flared exits are dimensioned such that at themaximum designed flow rate of combustible mixture the flame is retainedat the flame holder and such that at the lowest designed flow rate ofcombusible mixture the velocity of the combustible mixture within thenarrow portions of the passageways 52 to 58 is sufficient to prevent“flame back”, ie back propagation of the flame to the mixing chamber.

Also mounted insulatingly in the flame holder 30 is an ionization probe40 which again extends rearwardly through plate 12 to a terminal 42.Using ionization probe 40 and the ground electrode 38, the carboncontent of the flame can be monitored. If the carbon content is found tobe lower than a predetermined level, indicating inadequate combustion,the monitor can be arranged in known manner to trigger a control systemto interrupt the fuel supply. Thus, the flame can be extinguished.

In conventional blown gas burners, the gaseous fuel is ejected from anozzle at the end of the burner tube, and the flame is ignited at thatpoint. The gas is conveyed to the nozzle by an axially-disposed conduitinside the tube. The air required for combustion is supplied, by apowered air fan through ports in the tube, close upstream of the nozzle.The air mixes with the gas exiting the nozzle at the point of ignition.

For combustion to take place fully and stoichiometrically, air and gasmust be mixed together in the correct volumetric proportions. Where onegas is injected into the other, as in a conventional blown burner,combustion is not always at its most efficient, since mixing isoccurring while combustion is taking place. As a result, mixing of airand fuel is incomplete. It is virtually impossible to attain the correctair/fuel stoichiometry across the flame front. Thus, the flame isobserved to possess distinct, differently coloured flame zones,indicative of poor mixing, varying fuel/air stoichiometry and imperfectfuel combustion.

In contrast, with a burner according to this invention, the flameemanating from the flame holder 30 is observed to be substantiallyuniform across the entire flame front, uniformly bright blue and withvery little yellow flame regions being evident. A flame of thisappearance is a practical realisation of an ideal flame wherein the fuelis virtually completely combusted.

The complete combustion attainable by burner 10 is believed to be theresult of two features of the burner. First, the fuel and air areintroduced in the correct stoichiometric ratio governed primarily by thesizes of the bores 22, 24 of the nozzles 18, 20. Second, it will be seenfrom the drawing that the bores of nozzles 18, 20 introduce the air andfuel to the burner casing as counter flowing jets, i.e. the two jetsimpinge on one another. As shown, the nozzles providediametrically-opposed jets. Such impinging jets ensure very effectiveinitial mixing in the burner casing. Basically, highly turbulent flowsare created in the rearward end of the casing 11, which provides amixing chamber of significant length between the nozzles 18, 20 and theoutlet end of the flame holder 30. By the time fuel/air introduced bynozzles 18, 20 reach the flame holder 30, they are in a completely mixedcondition ideal for correct and complete combustion.

The operation and output of the burner 10 can be controlled in variousways. Desirably, the air supply will include a control valve and the airsupply line will incorporate a flow or pressure transducer. This, inturn, will control a fuel balancer, i.e. a gas booster or restricter.Such equipment will be known to the addressee and hence is not describedin detail here. Suffice to say, however, the objective of the controlsystem is to balance the gas and air pressures and flows to the burner10, to maintain the desired stoichiometry when turning down the burnerusing the air control valve. With such an arrangement, the only valve tobe operated is the air control valve.

Alternatively, referring now to FIG. 4, the burner could be controlledby a single valve (62) operating in the gas supply line instead. In thiscase, the gas pressure or flow is determined by a transducer (64) whichis used to control the air pressure or flow. By way of example, the airpressure or flow can be varied using a suitable variable speed fan orblower (66).

In installations utilising more than one burner, e.g. in a boilerhouse,it is contemplated that air and fuel gas will both be supplied at highpressure. Then, only balancer devices would be required to ensure allthe burners receive air and fuel in the correct volumetric ratios.

The burner 10 as described could be employed alone in a small appliance,e.g. a domestic or small commercial space heating system, or a cateringoven or grill. In larger systems for industry, a given furnace, boilerhouse, reactor or the like may require many such burners 10, which willmost conveniently be coupled to common air and fuel manifolds.

The burner 10 shown in the drawing burns remarkably quietly, thanks tothe highly stable flame. By way of example, one such burner has anoverall length of 275 mm and a diameter of 76 mm. The noise it generatesis less than that produced by a fan supplying the air required forcombustion.

What is claimed is:
 1. A burner for combusting a gaseous mixture ofgaseous fuel with a combustion supporting gas, such as oxygen or air,comprising a burner tube (11) open at one end (11′) and closed at itsother end (11″) with a flame holder (30) at which fuel is burnt adjacentthe open end (11′), the flame holder (30) being traversed by passageways(52, 54, 56, 58) for the gaseous mixture, the burner (10) having inlets(14, 16) adjacent the closed end (11″) connected to combustionsupporting gas and gaseous fuel supply lines, one of said lines having acontrol valve operable for controlling the size of the flame, the saidone line having a pressure or flow transducer and the other line havinga variable booster or restrictor responsive to the transducer, forbalancing air and fuel supplied to the burner (10) to ensure the gaseousmixture remains stoichiometric irrespective of the size of the flame andsuch that the lowest gaseous fuel mixture flow rate is at least as lowas {fraction (1/60)}^(th) the highest flow rate of the gaseous fuelmixture, each passageway (52, 54, 56, 58) having a flared exit (60) atthe end nearer the open end (11′) of the burner tube (11), eachpassageway being dimensioned such that at the highest obtainable flowrate of gaseous fuel mixture the flames do not lift off from the flameholder, and at the lowest flow rate the velocity of the gaseous fuelmixture at some point within the passageways (52, 54, 56, 58) issufficient to prevent flame back through the flame holder.
 2. A burneras claimed in claim 1 on which the inlets (14, 16) being furnished withmetering nozzles (18, 20) for separately delivering air and fuelnon-axially, e.g. substantially radially into the burner tube (11) whichforms a mixing zone between the inlets (14, 16) and the flame holder(30), the metering nozzles (19, 20) having orifices (22, 24) with theflow cross-sectional areas correlating to the stoichiometric ratio ofair-to-fuel for which the fuel is substantially completely burnt.
 3. Aburner according to claim 2, wherein the inlets (14, 16) are disposed inthe burner tube (11) for delivering air and fuel in directions whichimpinge, to create turbulence and mixing inside the tube.
 4. A burneraccording to claim 3, wherein the inlets (14, 16) are locateddiametrically opposite one another in the burner tube (11).
 5. A burneraccording to any one of claims 1 or 2, wherein the ratio of the flowcross-sectional areas of orifices (22, 24) is 10 to
 1. 6. A burneraccording to any one of claims 1 or 2, wherein the flame holder (30)provides a mounting for an igniter (32) and associated ground electrode(34).
 7. A burner according to claim 6, wherein the flame holder (30)further provides a mounting for an ionization probe (40) for detectingunburnt carbon in the flame.
 8. A burner according to claim 7, incombination with a monitor and control system coupled to the probe (40),in use for interrupting the fuel supply should the unburnt carbon exceeda predetermined level.
 9. A burner according to claim 8, wherein thevalve is in the air supply line and a booster or restricter is in thefuel line.
 10. A burner according to claim 9, wherein the valve is inthe fuel line and a variable speed fan is provided in the air line. 11.A burner as claimed in claim 1, comprising two or more radially nestedtubes each pair of adjacent tubes defining therebetween one of saidpassageways of the flame holder (30) for the gaseous fuel.
 12. A burneras claimed in claim 11, in which the tubes (30 a, 30 b, 30 c) are heldin position relative to each other by one or more transverse pins (33).13. A burner as claimed in claim 12, including a central bore with aflared exit.
 14. A burner as claimed in any one of claims 11, 12 or 13in which each flared exit has its terminal portion defined by inner andouter cylindrical walls which are parallel to the longitudinal axis ofthe flame holder.